Influence of saturated water content on estimating soil hydraulic properties from cumulative disc infiltrometer measurements

The soil sorptivity, S, and saturated hydraulic conductivity, K_s, are fundamental soil hydraulic properties that can be estimated from the cumulative infiltration curve measured with a disc infiltrometer. The Haverkamp infiltration model is widely used to estimate S and K_s. This model includes as inputs the constants β and γ and the difference between the initial, θ_i, and final, θ_s, volumetric water contents, Δθ. Since Δθ would be expressive of the possible measurement errors, and assuming β, γ, and θ_i as known values, the first objective of this work is to analyze the influence of θ_s on the optimization of K_s and S. To this end, a sensitivity analysis, which consists of estimating K_s and S for a range of θ_s was applied on synthetic infiltration curves simulated for homogeneous columns of sand and loam soil. Then, and working on real soils under different tillage management, we evaluated different procedures to measure θ_s and analyzed its impact on K_s and S estimation. Four different techniques were compared: the gravimetric-core method and two TDR invasive (3 and 5 cm) and a non-invasive (NiP) probes. All TDR probes were connected to a low-cost NanoVNA. The sensitivity analysis showed that θ_s, K_s and S can be optimized simultaneously from the inverse analysis of an infiltration curve when β and γ are known values and the infiltration curve is near the steady-state zone. However, due to the intrinsic complexities of real soils and the fact that β and γ are unknown variables, we recommended to optimize K_s and S using measured θ_s. The NiP sensor connected to a NanoVNA provided a fast, inexpensive, clean, accurate and robust alternative to measure θ_s at the end of the infiltration experiments.